This invention relates to apparatus for measuring electrochemical characteristics of a liquid, for example, the pH value of a blood sample.
The pH value of blood is a frequently measured physiological parameter which provides an indication of proper acid-base balance and blood-gas exchange through the lungs. In the field of obstetrics, pH tests are often made if there are indications of fetal distress during labor which may be caused by the fetus not receiving sufficient oxygen from the mother due to strangulation by the umbilical cord or premature separation of the placenta. Conventionally, the blood samples are taken by making a small incision on the fetal scalp and placing a capillary tube in the proximity of the incision whereby the blood is drawn up into the interior portions of the tube through capillary action.
One of the most commonly used clinical devices for measuring the blood pH is the blood-gas analyzer such as the pH blood-gas system manufactured by Corning Medical Corporation or the PHM71 Mk2 acid-base analyzer from Radiometer-Copenhagen. The pH sensor is of traditional construction using pH sensitive glass such as Corning Glass Works Type 01500 which has been formed into a cylindrical envelope in which is sealed an internal electrolyte and reference electrode. The sensing surface of the electrode is placed in contact with the blood, which in turn is contacting a reference electrode via an electrolyte junction. The system is maintained at a preselected temperature by way of a relatively complex circulating heated water bath. PG,3
These typical pH analyzers are calibrated at two pH values, generally 6.83 and 7.38, which allows adjustment for both electrode sensitivity and absolute potential. This two point calibration is somewhat complex necessitating relatively sophisticated electronics normally used by trained personnel.
Such analyzers generally require the transfer of the blood sample from the capillary tube to a receptacle in the instrument. As is known in the art, if any ambient air mixes with the blood sample, the pH value will change. Hence, there is a possibility of contaminating the blood sample during the transfer from the original collecting device to the instrument.
Since such analyzers are complex and costly devices, they are typically located only in the hospital laboratory where they are operated by skilled technicians. As a result, there is often considerable delay between the time of taking the blood sample until the results from the lab are received. Of course, such delays are undesirable in emergency situations.
U.S. Pat. Nos. 3,911,901 to Neidrach et al, 3,049,118 to Arthur et al, 3,399,667 to Nishimoto et al, 4,119,498 to Edwall et al and 3,892,652 to Levine et al disclose representative devices for measuring electrochemical activities of various samples. However, each of them, like the analyzers noted above, have drawbacks and do not readily lend themselves to fast, convenient and accurate pH measurements by relatively untrained personnel.
Many of these problems, however, are solved by the method and apparatus disclosed in commonly assigned U.S. patent application Ser. No. 965,901, now U.S. Pat. No. 4,272,245, to Diamond et al which discloses the broad concept of utilizing a disposable cartridge containing indicating and reference electrodes. A common electrolytic solution is used to establish a one point calibration factor and also serves as an electrolytic bridge during measurement of the sample. The sample is collected in a capillary tube and the cartridge is adapted to receive the blood sample directly from the tube. The capillary tube is slipped over the indicating electrode so that the blood displaces the solution and covers the indicating electrode. The electrical properties between the two electrodes are again measured and used to calculate the electrochemical activity (e.g. pH) of the sample in conjunction with the previously measured calibration factor. This application is hereby incorporated by reference.
The present invention is directed to improvements to this general concept. More particularly, an entire system is designed pursuant to various features of this invention that permits fast, convenient, and accurate measurements of electrochemical activity of liquid samples without requiring the need for trained personnel to operate the equipment.